hair extensions
Hair extensions don't damage your hair at all. There are lots of different types you can get including temporary clip-ins, clip-ins that are sewn into your hair, micro-rings (your own hair and an extension is threaded through a cute tiny bead that is then tightened with pliers, and fusion extensions.
Fusion extensions can only be used on long hair (at least shoulder length) or else the extensions will show. Ick.
You could also consider a weave which looks real and is fairly long-lasting. It's just sewn into the hair and also doesn't damage it at all.
Check out my journal entries under Bi-atch. I have a few entries about extensions and weaves.